In the melt extrusion molding of a thermoplastic resin sheet, a shaping roll for melt extrusion molding that has a fine structure on its surface is used to transfer the fine structure to the surface of the sheet, so that various functions are given to the surface of a sheet molded article formed of thermoplastic resin. When a sheet molded article of thermoplastic resin having a fine structure on its surface is formed using melt extrusion molding, the transfer uniformity in the width direction, especially the transfer uniformity in the end part of the sheet molded article, is a major problem.
Generally, a sheet (often referred to below as a “sheet”) of molten thermoplastic resin having flowed from the lip part of a T-die or coat hanger die is cooled by air in an air gap and the sheet temperature is significantly reduced before being pressed between a metal shaping roll (often referred to below as a “first roll”) for melt extrusion molding and a pressing roll (often referred to below as a “second roll”). Since the degree of reduction of the temperature significantly differs between the sheet middle part and the sheet end part and the part of the sheet closer to the sheet end part is easily cooled, reduction of the temperature is larger in the part closer to the sheet end part. Accordingly, the transfer state of the fine structure of the surface of the first roll differs between the sheet middle part and the sheet end part, thereby producing non-uniformity in physical properties (for example, birefringence and thickness) between the sheet middle part and the sheet end part.
In a method for preventing non-uniformity in the transfer state and physical properties in the sheet width direction, temperature changes along the axial line of the first roll of the sheet in contact with the first roll are made different. Normally, the first roll internally has a heat medium passage for controlling the surface temperature of the first roll. When the distance from the heat medium passage to the surface of the first roll is short, the temperature changes of the sheet in contact with the first roll is larger than in the case in which the distance from the heat medium passage to the surface of the first roll is long. Accordingly, when the distance from the heat medium passage to the surface of the first roll in the vicinity of the end part of the first roll is longer than the distance from the heat medium passage to the surface of the first roll in the middle part of the first roll, the temperature changes of the part of the sheet in contact with the end part of the first roll can be made smaller than the temperature changes of the part of the sheet in contact with the middle part of the first roll. As a result, the transfer state and physical properties of the fine structure of the surface of the first roll can be made uniform between the sheet middle part and the sheet end part.
For example, the structure disclosed in FIGS. 11 and 12 of Japanese Unexamined Utility Model Application Publication No. 1994-001834 can reduce the distance from the heat medium passage to the surface of the first roll in the middle part of the first roll and increase the distance from the heat medium passage to the surface of the first roll in the end part of the first roll.